I have collected some data to compare the biodiversity of a field in which the plagioclimax is maintained by machine mowing with a field in which the plagioclimax is maintained by sheep grazing.

What I found was that there was a significant increase in floral biodiversity in the field which was managed by agricultural grazing when compared to the floral biodiversity of the machine-mown field.

I am trying to explain my findings, and I believe that it is due to the comparitively short amount of time that machine-mowing has been a grassland management technique, so there has been little time for natural selection to take place and for species to develop adaptations to machine-mowing (such as being low-lying, or extremely fast-growing). Whereas the field which is managed through sheep-grazing, contains multiple competing species which have adapted over time to grazing sheep, such as Cirsium arvense.

Moreover, the field which is managed by grazing has nutrients returned to it through defecation of the grazing animals, allowing for more favourable conditions for growth.

Is my explanation valid? Are there any other reasons why the floral biodiversity of a field managed by sheep grazing would be higher than one managed by machine-mowing?

2 Answers
2

I think you may have covered the answer in your question... but here's my take;

The 'mowing' occurs at a distinct time-point and is highly destructive and uniform, so the selection pressure is very high and only highly competitive species that are fast to grow and reproduce will be successful.

As you point out, animals will defecate and naturally fertilize the land, thus favouring growth. Other favourable factors in an agricultural environment will be that the 'mowing' will be much less uniform; the animals may favour one plant over another - so a plant that is not eaten by the animals will continue to grow and reproduce, whereas in the mowing environment this plant would not succeed; this will allow various other species to succeed where they would not in the mown environment. Also, depending how many animals are in the area the height of the grass may be higher overall, so slower-growing plants could grow enough to reproduce.

Different agriculture techniques. I could imagine machine-mowed grassland could be processed by different chemicals (fertilizers, insecticides). I could imagine machine-mowed grassland could be sown with different grass that grows faster but dies when trampled with hoofs.

Most agriculture machinery works on fossil fuel i.e. emits CO₂ and other compounds.

You could also flip cause and effect. It could be that there're some reasons (dunno: soils, humidity, etc) why fields with lower biodiversity is more suitable for mowing.